Handbook of Solvents - George Wypych - ChemTech - Ventech!

906 Martin Hanek, Norbert Löw, Andreas Mühlbauer
along the glue boundaries. Secondly, the glue itself, which in most cases is a cured epoxy
material, may dissolve in the cleaning medium. The combination of these two factors can
lead to severe stencil defects. A protective coating can be applied to the glue in cases where
it is known that the glue is not stable towards the cleaning medium. Advanced cleaning
agent manufacturers work closely with stencil manufacturers to avoid this problem. However,
testing in advance of installed cleaning operation ensures that no unexpected stencil
defects will affect the production process.
14.8.3.3 Different cleaning media
When selecting a cleaning process, the chemistry should be selected first, because the cleaning
chemistry has to be adopted to the chemical ingredients of the contamination. Only after
the evaluation of the proper chemistry, can the method of applying the cleaning agent be
evaluated.
Solvent-based cleaning agents such as hydrocarbons, alcohols, terpenes, and esters offer
properties that make them effective cleaning agents for the electronics industry. Low
surface tension, high solubility, and ease of drying allow organic solvents to clean effectively
where water-based cleaning agents are less effective. These benefits are very similar
to the physical properties of the chlorofluorocarbons (CFC) that were once in widespread
use in electronics manufacturing operations. Solvents, however, have several significant
drawbacks that have limited their use including:
• Flame and explosion hazards,
• Higher equipment costs, due to required explosion protection,
• Personal exposure concerns,
• Material costs, and
• Disposal of used solvents.
And while water-based cleaning agents have been much improved in recent times,
their performance does not always match that of their solvent forerunners. In fact, an ideal
water-based cleaning agent would combine the physical properties of solvent-based cleaning
agents with the safety and convenience of water-based materials.
Solvent-based cleaning agents
Cleaning with solvents such as isopropanol or acetone still remains the most widely applied
method because any type of solder paste or SMT adhesive can be removed from the stencils
by specially formulated mixtures. Consequently, the cleaning process window for such
modern solvents is sufficiently wide to ensure the required results when changing over to
another paste or adhesive. Normally, such cleaning processes are performed in machines
designed specifically for solvent cleaning. The use of cleaners with flash points in excess of
104ºF (40°C) means that the machines can be set up without having to maintain an explosion
protected area. Moreover installation in a special room may be necessary. However,
the handling requirements of highly flammable cleaners, for such as isopropanol (IPA, flash
point of 54ºF, 12°C), are far more critical, and cleaning is not so effective, particularly with
fine-pitch stencils and SMT adhesives. Besides, modern solvent cleaners have been formulated
in such a manner that health hazards are minimized - provided, they are properly used.
Water-based cleaning agents
The benefits of cleaning with water-based agents are straightforward. They include:
• Ease of use,
• Absence of fire or explosive hazards,
• Simple disposal,